Hybrid vehicles come in different forms, may use different energy storage devices, and serve different customer needs. Existing hybrid vehicles include hybrid electric vehicles (HEVs), which use batteries as the energy storage system. The plug-in hybrid electric vehicle (PHEV) is an extension of existing hybrid electric vehicle (HEV) technology. A PHEV utilizes a larger capacity battery pack than a standard hybrid vehicle, and adds the capability to recharge the battery from a standard electrical outlet to decrease fuel consumption and to further improve the fuel economy in an electric driving mode or in a blended driving mode. There are also battery electric vehicle (BEV) applications where an electric machine completely replaces the internal combustion engine.
In most hybrid electric vehicles (HEVs), only a small portion of the battery capacity is generally used, therefore capacity may be assumed constant. In a plug-in hybrid electric vehicle (PHEV) or battery electric vehicle (BEV), battery capacity fade may have a significant impact. Battery state of charge (SOC) is defined as percentage of available charge as compared with the maximum charge capacity. For example, the well-known SOC calculation using amp-hour integration in part depends on a predetermined battery capacity:
                              S          ⁢                                          ⁢          O          ⁢                                          ⁢                      C            ⁡                          (                              k                +                1                            )                                      =                              S            ⁢                                                  ⁢            O            ⁢                                                  ⁢                          C              ⁡                              (                k                )                                              -                                    T              s                        *                                          η                *                                  I                  ⁡                                      (                    k                    )                                                              Q                                                          (        1        )            where SOC is the state of charge, Ts is the time interval, η is the charge/discharge efficiency, I is the current, and Q is the capacity.
For the foregoing reasons, there is a need to infer battery capacity based on battery behavior and data.
Background information may be found in US 2008/0054850 A1, EP 1873542 B1, WO 06057468 A1. Further background information may be found in: Plett, Extended Kalman filtering for battery management systems of LiPB-based HEV battery packs Part 3. State and parameter estimation, Journal of Power Sources 134, pages 277-292, Elsevier, 2004; Tate, Jr. et al., Shortest path stochastic control for hybrid electric vehicles, International Journal of Robust and Nonlinear Control 18, pages 1409-1429, Wiley, 2007.